This application is related to Korean Application No. 99-40289, filed Sep. 18, 1999 and Korean Application No. 00-41921, filed Jul. 21, 2000, the disclosures of which are hereby incorporated herein by reference.
The present invention relates methods for measuring semiconductor wafers, for example, during the manufacture of a semiconductor device, and more particularly, methods for measuring semiconductor wafers which have undergone an over-etching process.
When manufacturing semiconductor devices, a dry etching type of contact etching process using over-etching may be applied to form, for example, a direct contact (DC) or a buried contact (BC). Such process may sufficiently etch oxide taking into account the uniformity of the etching process itself. A single crystalline silicon substrate (a wafer) may be over-etched during an etching process, but poly crystalline silicon, not the silicon substrate, is typically over-etched during an etching process for forming a BC, and metal is typically over-etched during a process of forming a metal contact (MC).
After an over-etching process has been performed, the completion of the over-etching process is generally determined based on presence/absence of oxide remaining at an oxide site and the measured thickness of the remaining oxide. Typically, when the film structure of an oxide site is inspected after an over-etching process, it can be seen that oxide is over-etched down to a silicon substrate. Accordingly, the success or failure of over-etching generally can be determined by measuring the thickness of the remaining oxide at an oxide site that has undergone an over-etching process.
However, most conventional measuring equipment has been considered unsuitable for monitoring an over-etching process because an under-etched state of a film typically cannot be exactly discriminated from an over-etched state of a film based on a value measured by the measuring equipment.
FIG. 1 is a graph showing the results of measuring the thickness of remaining oxide (Tox) using an optical measuring apparatus with respect to wafers which are obtained after dry-etching active silicon nitride (SiN) during manufacture of 64-M dynamic random access memory (DRAM) devices. In the process of accumulating measurement data, one wafer is sampled on each day. For example, as shown in FIG. 1, the measured values of the thickness of remaining oxide at an oxide site which has undergone dry etching of SiN are uniformly maintained in a range of 0-5 xc3x85 (angstroms). Based on the fact that the measured values occupy a very small range, an operator can generally determine whether or not an over-etching process is performed normally and successfully.
FIG. 2 is a graph showing the results of measuring the thickness of remaining oxide with respect to wafers which are obtained after over-etching the oxide for forming a DC during manufacture of 64-M DRAM devices. In the process of accumulating measurement data, five wafers are sampled on each day. FIG. 2 shows the measured results with respect to wafers on which over-etching has been completed based on actual measurement.
As shown in FIG. 2, despite the actual successful over-etching of a wafer during an oxide etching process for forming a DC, the measured results do not always allow an operator to determine that the over-etching is successfully completed. As shown in FIG. 2, the measured values of the thickness of remaining oxide on a wafer on which over-etching is actually completed are distributed throughout a range of 0-100 xc3x85. Accordingly, when a measured value is about 90 xc3x85, that is, when it is determined that oxide of a thickness of about 90 xc3x85 remains, it is difficult to determine the success or failure of over-etching based on this value.
As the measured results of conventional measuring equipment may be inaccurate, conventionally, the thickness of remaining oxide detected when over-etching is successfully achieved, is set to within a very wide range of 0-400 xc3x85 in the case of an over-etching process for forming a DC, and to within an even wider range of 0-1000 xc3x85 in the case of an over-etching process for forming a MC. Accordingly, oxide that is not actually etched within this range may not be monitored, thereby potentially causing process failures.
Embodiments of the present invention include methods for measuring a semiconductor wafer which has been subjected to an etching process. Light is radiated at the semiconductor wafer. Light within a selected wavelength band reflected from the semiconductor wafer is measured to provide an output value. A ratio of the output value and a reference value is determined. The reference value may be based on light within the selected wavelength band reflected from a reference surface, such as a bare silicon reference surface. It is determined that the semiconductor wafer is under-etched if the determined ratio does not meet the reference value. A normalized optical impedance or a polarization ratio may be measured based on light within a selected wave length band reflected from the semiconductor wafer to provide the output value in various embodiments of the present invention. In further aspects of the present invention, a thickness of a remaining oxide layer is determined using an under-etch recipe when it is determined that a semiconductor wafer is under-etched and a thickness of a damaged/polymer layer may be determined using an over-etch recipe when it is determined that the semiconductor wafer is over-etched.
In further embodiments, the present invention provides methods of determining the etched state of a semiconductor wafer. The methods include the steps of radiating light at bare silicon and obtaining a reference value from an electrical signal generated by light within a predetermined wavelength band of the light reflected from the bare silicon, radiating light within the predetermined wavelength band at a target of measurement, i.e., a wafer that has undergone an over-etching process, obtaining an output value corresponding to the reference value from an electrical signal generated by light within the predetermined wavelength band of the light reflected from the wafer, calculating the ratio of the output value to the reference value, and comparing the calculated ratio with a predetermined reference ratio to determine whether or not the wafer is under-etched.
In other embodiments, the present invention provides methods of determining the etched state of a semiconductor wafer. The methods include the steps of radiating light at bare silicon and obtaining a reference value by integrating an electrical signal generated by light within a predetermined wavelength band which is reflected from the bare silicon, radiating light within the predetermined wavelength band at a target of measurement, i.e., a wafer that has undergone an over-etching process, obtaining an output value by integrating an electrical signal of the predetermined wavelength band of light reflected from the wafer, calculating a ratio of the output value to the reference value, and comparing the calculated ratio with a predetermined reference ratio to determine whether or not the wafer is under-etched.
In yet further embodiments, the present invention provides methods of measuring the etched state of a semiconductor wafer. The methods include a first step of radiating light within a predetermined wavelength band at a wafer that has undergone a plasma over-etching process, a second step of obtaining a predetermined output value from an electrical signal corresponding to light reflected from the wafer, a third step of determining whether an optical impedance of the light reflected from the wafer changes based on the output value, a fourth step of determining whether the over-etching process is successfully completed depending on the change in the optical impedance determined in the third step, a fifth step of measuring the thickness of a wafer film according to a recipe employing a predetermined over-etching completed film stack when it is determined that the over-etching process is successfully completed, and a sixth step of measuring the thickness of a wafer film according to a recipe employing a predetermined over-etching incompleted film stack when it is determined that the over-etching process is not successfully completed.
In still other embodiments, the present invention provides methods of measuring the etched state of a semiconductor wafer. The methods include a first step of radiating light at bare silicon and obtaining a reference value 1 from an electrical signal generated by light within a predetermined wavelength band among the light reflected from the bare silicon, a second step of radiating light within the predetermined wavelength band at a target of measurement, i.e., a wafer that has undergone an over-etching process, a third step of obtaining an output value 1 corresponding to the reference value 1 from an electrical signal of light within the predetermined wavelength band among the light reflected from the wafer, a fourth step of calculating a ratio of the output value 1 to the reference value 1, a fifth step of comparing the calculated ratio with a predetermined reference ratio to determine whether or not the wafer is under-etched, a sixth step of measuring the thickness of remaining oxide using a predetermined under-etch recipe when it is determined that the wafer is under-etched, a seventh step of obtaining a predetermined output value 2 from an electrical signal corresponding to light reflected from the wafer when it is determined that the wafer is not under-etched, an eighth step of determining whether an optical impedance to the light reflected from the wafer changes based on the output value 2, a ninth step of determining whether or not the over-etching process is successfully completed depending on the change in the optical impedance determined in the above step, a tenth step of measuring the thickness of a wafer film according to a recipe employing a predetermined over-etching completed film stack when it is determined that the over-etching process is successfully completed, and an eleventh step of measuring the thickness of a wafer film according to a recipe employing a predetermined over-etching incompleted film stack when it is determined that the over-etching process is not successfully completed.
In yet further embodiments, the present invention provides methods of measuring the etched state of a semiconductor wafer. The methods include a first step of radiating light within a predetermined wavelength band at a wafer that has undergone a plasma over-etching process, a second step of obtaining predetermined optical impedances 1 and 2 from an electrical signal corresponding to light reflected from the wafer, a third step of comparing the optical impedance 1 with a predetermined reference impedance 1 to determine whether or not the wafer is completely over-etched, a fourth step of measuring the thickness of a wafer film according to a recipe employing a predetermined over-etching completed film stack when it is determined that the over-etching process is successfully completed in the third step, a fifth step of comparing the optical impedance 1 with a predetermined reference impedance 1 to determine whether or not the wafer is completely over-etched when it is determined that the over-etching process is not completed in the third step, a sixth step of measuring the thickness of a wafer film according to a recipe employing a predetermined over-etching completed film stack when it is determined that the over-etching process is successfully completed in the fifth step, and a seventh step of measuring the thickness of a wafer film according to a recipe employing a predetermined over-etching incompleted film stack when it is determined that the over-etching process is not successfully completed.
The reference ratio may be 92%, and the wavelength band may be 190-826 nm. The reference value is obtained from a normalized output value corresponding to an electrical output signal generated by light within the above wavelength band of the light reflected from the bare silicon, and the output value may be obtained from a normalized output value corresponding to an electrical output signal generated by light within the wavelength band of the light reflected from the wafer.
The over-etching incompleted film stack may be set to [oxide/silicon], and the over-etching completed film stack may be set to [polymer/oxide/damaged layer/silicon]. The thickness of the oxide may be set to 0. In the measuring step using the film stack, the thickness of the oxide need not be measured, but the thickness of the damaged layer and the polymer may be measured, because the oxide and the polymer typically have similar optical properties, and because if over-etched the oxide does not remain, but the polymer may be present. A damaged layer may indicate a damaged silicon layer formed on the surface of the wafer by plasma during the etching process.
The thickness of the oxide remaining on the silicon may be calculated from the electrical signal according to a recipe in which the film stack is set to [oxide/silicon]. The thickness of the damaged layer and the polymer on the silicon may be calculated from the electrical signal according to the recipe in which the film stack is set to [oxide/damaged layer/silicon] or [oxide/polymer/damaged layer/silicon].
In further embodiments of the present invention, the methods include obtaining a difference value (AR) between a maximum reflectivity (Rmax) and a minimum reflectivity (Rmin) from the electrical signal, and comparing the difference value (xcex94R) with a predetermined reference difference value to determine whether the target of measurement is over-etched. The reference difference value may be set to 0.01. An electrical output signal corresponding to light reflected from the wafer may be normalized based on a particular output signal from the bare silicon which may be set to xe2x80x9c1xe2x80x9d so that the electrical output signal can be used as a reference value.